This application is a continuation application filed under 35 U.S.C. 111(a) claiming the benefit under 35 U.S.C. 120 and 365(c) of a PCT International Application No. PCT/JP2003/006081 filed May 15, 2003, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
The PCT International Application No. PCT/JP2003/006081 was published in the English language on Nov. 25, 2004 under International Publication Number WO 2004/102539 A1.
1. Field of the Invention
The present invention generally relates to magnetic storage media and magnetic storage apparatuses, and more particularly to a longitudinal magnetic recording medium having a seed layer and suited for a high-density recording, and to a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
A typical longitudinal magnetic recording medium includes a substrate, a seed layer, a Cr or Cr alloy underlayer, a CoCr alloy intermediate layer, a Co alloy magnetic layer where the information is written, a C overlayer, and an organic lubricant, which are stacked in this order. Substrates that are being presently used include NiP-plated AlMg alloy substrates and glass substrates. The glass substrate is more popular due to its resistance to shock, smoothness, hardness, light weight, and minimum flutter especially at a disk edge in the case of a magnetic disk.
In a first example shown in FIG. 1, on a glass substrate 1 is formed an amorphous layer 3 made of NiP. The NiP layer 3 is preferably oxidized. On the NiP layer 3 is grown an underlayer made up of two essentially Cr underlayers 4 and 5 with a (002) texture on which a magnetic layer 7 is deposited. The second Cr underlayer 5 usually has a larger lattice parameter than the first Cr underlayer 4. The magnetic layer 7 has a (11 20) crystallographic orientation, and may be made up of a single layer or multiple layers that are in direct contact and behave magnetically as one magnetic layer. An interlayer 6 made of a CoCr alloy may be disposed between the magnetic layer 7 and the Cr underlayers 4 and 5. To enhance the adhesion of NiP to glass, elements such as Cr may be alloyed with NiP or a separate adhesive layer 2 made essentially of Cr may be employed. However for metallic substrates like Al, it is not required to employ this adhesive layer 2. On the magnetic layer 7, a protective layer 8 made of C, and an organic lubricant layer 9 are deposited for use with a magnetic transducer such as a spin-valve head.
In a second example shown in FIG. 2, the structure is similar to that of FIG. 1. But in FIG. 2, the magnetic layer 7 is replaced by a plurality of layers 7-a and 7-b that are antiferromagnetically coupled through a spacer layer 10 made of Ru, so as to form the so-called synthetic ferrimagnetic medium (SFM). The first layer 7-a functions as a stabilizing layer, and the second layer 7-b functions as a main recording layer.
A third example shown in FIG. 3 utilizes a refractory metal seed layer 3-a made of Ta-M, where M is either nitrogen or oxygen. On the glass substrate 1 is formed a Ta-M seed layer 3-a either by reactive sputtering with Ar+N2 or Ar+O2 gas on which an underlayer 4 is deposited. The crystallographic orientation of (002) is mentioned in a U.S. Pat. No. 5,685,958 to Funtamoto et al., but the composition of the underlayer is limited to Cr or Cr alloy, and no mention is made of underlayers made of materials such as B2 structured materials, for example. The magnetic layer 7 is formed on the interlayer 6 or the underlayer 5 with a (11 20) preferred orientation as in the first example described above.
The microstructure of the magnetic layer, which includes grain size, grain size distribution, preferred orientation and Cr segregation, strongly affects the recording characteristics of the magnetic recording medium. The microstructure of the magnetic layer is usually controlled by use of one or more seed layers and one or more underlayers, with suitable mechanical texturing of the substrate. Small grain size and small grain size distribution with excellent crystallographic orientation are desired for extending the longitudinal magnetic recording technology for the current densities on the order of 50 Gbits/in2) and for the future recording densities of 100 Gbits/in2 or greater.
A seed layer is usually closest to the substrate and aids primarily in promoting a desired crystallographic orientation on the subsequent layers such as an underlayer. The seed layer is most likely amorphous and made of NiP or B2 structured materials, while the underlayer is most likely crystalline and made of bcc structured materials such as Cr and has either a (002), (110) or (112) texture.
The most extensively used underlayer has been Cr or Cr alloys with Mo, Mn, V, Ti or W, where the Cr content is typically at least 70 at. % and the additives are most often for increasing the lattice parameter. This lattice parameter increase helps to reduce the lattice mismatch between the Cr underlayer and the Co alloy magnetic layer. These are usually deposited on mechanically textured or non-textured Ni81P19 on suitable substrates like glass or Al. Mechanical texturing invariably exposes NiP to air which oxidizes the film surface. Oxidation is important for the Cr to grow with a (002) in-plane texture which results in the subsequently deposited magnetic layer to have a (11 20) crystallographic texture.
This is taken advantage of by a U.S. Pat. No. 5,866,227 to Chen et al., where a reactively sputtered NiP (with O2) seed layer on a glass substrate is described. Typically, Cr is deposited at a substrate temperature Ts>180° C. to promote a (002) texture with no (110) peak in the XRD spectrum. Deposition at low temperature Ts may result in smaller grains but a (110) texture is developed. NiP does not adhere very well to glass, and an adhesive layer such as that described in a U.S. Pat. No. 6,139,981 to Chuang et al. can be used. On NiP seed layers, underlayer grain sizes on the order of 8 nm to 10 nm can be realized by using two Cr alloy layers and by reducing the total underlayer thickness to less than 10 nm. Increasing the total thickness tends to significantly increase the average grain size. For example, for a single layer of Cr80Mo20 with a thickness t=30 nm, the average grain size can be approximately 20 nm which is obviously inadequate for present day media noise requirements.
Tang et al., “Microstructure and texture evolution of Cr thin films with thickness”, J. Appl. Phys., vol. 74, pp. 5025-5032, 1993 also observed grain diameter increase with thickness. To achieve an average grain size less than 8 nm is difficult as further reduction of the underlayer thickness results in degradation of the magnetic layer c-axis in-plane orientation (IPO). Although the underlayer average grain size can be small, a few large grains occasionally occur on which two or more magnetic grains may grow. The effective magnetic anisotropy of such grains may be reduced if magnetic isolation is not complete.
Alternate approaches to reduce grain size include inclusion of B (boron) onto the CoCrPt matrix. B inclusion reduces the grain size of recording layer and substantially improves the signal-to-noise ratio and magnetic properties of the magnetic recording medium. However, adding very high percentage of B increases the fct phase and hence the crystallographic quality goes bad beyond a certain B percentage, especially over B concentration of 8%.
A U.S. Pat. No. 5,693,426 to Lee et al. describe ordered alloy underlayers with the B2 structure such as NiAl and FeAl. Ordered alloys with structures such as B2, L10 and L12 are expected to have small grain sizes presumably due to the strong binding between the component atoms. Both NiAl and FeAl grow on glass substrates with a (211) texture which makes the magnetic layer c-axis to be in-plane with a (1010) texture. Grain sizes on the order of 12 nm can be achieved even for thick layers greater than 60 nm. The use of both NiAl and Cr on NiP has also been proposed in a U.S. Pat. No. 6,010,795 to Chen et al. In this case, NiAl develops a (001) texture due to the (002) texture of the crystalline Cr pre-underlayer and the magnetic layer texture is Co(11 20).
There are other seed layers aside from NiP that promote a Cr(002) texture. A U.S. Pat. No. 5,685,958 to Futamoto et al. propose refractory metals such as Ta, Cr, Nb, W and Mo with a reactive element consisting of at least 1% nitrogen or oxygen. In the case of Ta, which is reactively sputtered with Ar+N2 gas, as the N2 volume fraction is increased, Cr(002) appears in the XRD spectrum as well as Co(11 20). A typical underlayer thickness of 50 nm was mentioned and wide variations in the thickness were claimed to only slightly affect the media magnetic characteristics. But as the volume fraction is increased to 3.3%, both peaks disappear, indicating the degradation of crystallographic orientation. Futamoto et al. propose a useful range of substrate temperatures Ts of 150° C. to 330° C. and a more preferred range of 210° C. to 250° C. This would make the substrate temperature Ts necessary for the deposition of the Cr onto TaN similar to that onto NiP. A useful range of nitrogen partial pressure of 0.1 mTorr to 2 mTorr was also suggested. The nitrogen concentration of the Ta—N films are unknown but may be between 10 at. % to 50 at. %.
Kataoka et al., “Magnetic and recording characteristics of Cr, Ta, W and Zr pre-coated glass disks”, IEEE Trans. Magn., vol. 31, pp. 2734-2736, 1995 have earlier reported Cr, Ta, W, and Zr pre-coating layers on glass. For Ta films, reactive sputtering with the proper amount of N2 actually improves the succeeding Cr underlayer crystallographic orientation. Cr directly deposited on glass develop not only the preferred (002) orientation but also an undesirable (110) texture.
Oh et al., “A Study on VMn underlayer in CoCrPt Longitudinal Media”, IEEE Trans. Magn., vol. 37, pp. 1504-1507, 2001 reported a VMn alloy underlayer where the V content is 71.3 at. % and Mn content is 28.7 at. %. V has a high melting point of approximately 1500° C. and in principle may grow with small grains when sputtered but the texture is a very strong (110) on glass and on most seed layers.
A U.S. Pat. No. 5,344,706 to Lambeth et al. also proposed polycrystalline seed layers such as MgO which is B1 structured and a myriad of B2 structured materials such as NiAl and FeAl which function as templates for the succeeding Mn-containing alloys.
Good IPO leads to an increase in remanent magnetization and signal thermal stability. Goo IPO also improves the resolution or the capacity of the magnetic recording medium to support high-density bits. Recently developed synthetic ferrimagnetic media (SFM) provide improved thermal stability and resolution compared to conventional magnetic recording media of the same remanent magnetization and thickness product Mrt. The SFM is proposed in Abarra et al., “Longitudinal recording media with thermal stabilization layers”, Appl. Phys. Lett., Vol. 77, pp. 2581-2583, October 2000. Seed layers that can be used for conventional magnetic recording media can also be used for the SFM, but the potential of the SFM for extending the limits of longitudinal recording can best be realized if the IPO is close to perfect.
The IPO can be quantified by low incident angle XRD such as that made by Doerner et al., “Demonstration of 35 Gbits/in2 in media on glass substrates”, IEEE Trans. Magn., vol. 37, pp. 1052-1058, March 2001 or, more simply, by taking the ratio of the coercivity normal to and along the film plane (h=Hc⊥/Hc, where Hc⊥ denotes perpendicular coercivity, and Hc denotes coercivity along the film plane).
The ratio h for the magnetic recording media on Cr(002)/NiP is typically h≦0.15, and h>0.2 is observed only for badly lattice matched underlayers and magnetic layers. For h≦0.15, the M(H) hysteresis loop perpendicular to the film normal is approximately linear with field and Hc⊥ is typically less than 500 Oe. In the case of NiAl, the (211) texture is weak and thicknesses greater than 50 nm are usually needed to realize the above and reduce the occurrence of magnetic grains with a (0002) orientation. Previous work on using NiAl directly on glass as a seed layer for conventional magnetic recording media resulted in poor squareness (h>0.25) and could not match the performance of magnetic recording media on Cr(002)/NiP. This is the case even when seed layers such as NiP and CoCrZr are employed.
XRD measurements by Doerner et al. showed that the magnetic c-axes are spread over an angle greater than ±20° compared to less than ±5° for magnetic recording media on NiP/Al—Mg substrates. For magnetic recording media on TaN, though the Cr(002) and Co(11 20) peaks are visible from the XRD data, h>0.2 and the magnetic recording media underperform magnetic recording media on Cr(002)/NiP. The Cr alloy underlayer thickness used here is less than 10 nm, and the reduction of the ratio h was not observed by further increases in the underlayer thickness to greater than 20 nm.
Aside from the IPO, another concern in the manufacturing of the SFM is the increase in the number of chambers necessary compared to conventional magnetic recording media especially when bare glass substrates are used. Moreover, as throughput has to be maintained at a high level, the thickness of the deposited film is limited typically to 30 nm. Seed layers or underlayers that need to be thicker will require two chambers. The typical sequential deposition must also be made in a rapid fashion not only to have a high yield but also to prevent the temperature of the high emissivity glass substrate to drop before the magnetic layers are deposited. Else, a heating step is needed which will require a separate process chamber. The substrate emissivity is decreased by the seed layer and the underlayer such that both cannot be very thin. If a bias voltage is to be applied as in CVD C deposition, the total magnetic recording medium thickness needed is usually greater than 30 nm.
Recently, there have been studies on B2 structured AlRu seed layers on glass, and AlRu was found to be an excellent material in use with glass substrates for substantial improvement of the IPO over NiP coated glass substrates or NiAl coated glass substrates. However the useful range of AlRu where this is applicable is where the Ru content is 50% and AlRu has the B2 structure. Since the Ru content is large, the cost of the target is very high. In the current magnetic recording media, double AlRu layers are used with approximately 25 nm each to get the good IPO and signal-to-noise ratio requirements.